After crossing off the "obvious" possibilities (that is, eliminating from the row and column all the numbers that are already used) you should find that in that -x- spot you have only 3 and 9 as options.

BUT, in that fourth row, there's nowhere else the 3 can go! There are already 3s in all the other columns.

Looking at that column 8, the topmost thing can only be 6 or 9. Also in row 7, that spot can only be 6 or 9. Therefore, among those two spots, the 6 and the 9 get used up: 6 and 9 must be the 1st and 7th things in column 8. Therefore no other thing in column 8 can be 6 or 9. Does that make sense?